Allow 0.0.0.0 and :: as default gateways

On point-to-point interfaces, some RILs return the unspecified
address (0.0.0.0 or ::) as the gateway. This is correct, as
routes pointing at point-to-point interfaces don't need
gateways. Currently, telephony ignores these routes, causing
loss of connectivity (for example, when using an IPv6 APN on
Galaxy Nexus). Allow these routes to be set.

Bug: 8475171
Bug: 8276725
Change-Id: I3a770628c9717bb6e451a523417401b56ba55763